Methods for Evaluating Soybean Cultivars for Resistance to Sclerotinia sclerotiorum. Molly Niedbalski Cline, Postdoctoral Extension Associate, Department of Plant Pathology, University of Illinois at Urbana-Champaign, Urbana 61801. B. J. Jacobsen, Associate Professor, Department of Plant Pathology, University of Illinois at Urbana-Champaign, Urbana 61801. Plant Dis. 67:784-786. Accepted for publication 15 December 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-784.

In several areas of Illinois, stem rot or white mold caused by Sclerotinia sclerotiorum has become a problem in soybean (Glycine max) fields previously cropped to snap beans (Phaseolus vulgaris). The disease reaction of soybean cultivars from maturity groups II–IV was evaluated by three inoculation methods. Plants in full bloom (R2) were inoculated by spraying an ascospore suspension (1–5 × 10⁵ ascospores per milliliter) to runoff. Four-week-old prebloom plants (V4-V5) were inoculated by placing pieces of autoclaved carrot colonized by the fungus directly onto the leaf surface. After inoculation with either ascospores or colonized carrot pieces, plants were placed in a mist chamber for 7–12 days at 20–25 C with a 12-hr photoperiod. Reactions of all cultivars tested by these two inoculation methods were highly susceptible. Ascospores readily infected plants in the blossom stage and only infected 4-wk-old plants when an exogenous nutrient source was added or when tissues were injured. Limited-term inoculation, the third method, was accomplished by attaching autoclaved celery pieces colonized by the fungus to the nodes of 4-wk-old plants (V4-V5) for 24 hr. Differences in susceptibility were detected among 10 soybean cultivars evaluated by this method.